1. Field of the invention
The present invention relates to a device and a method for feeding a sheet or sheet-like material, for example, a film which is used in a flat-bed type scanner, an image setter for producing a printing prepress, and the like, in order to record by exposure.
2. Description of the Prior Art
An example of a flat-bed type scanner having a conventional sheet feeding device is shown in FIG. 1. A magazine 4 houses a roll film 6 made by winding up a film 2 which is a kind of sheet. A film 2 rewound from the roll film 6 is fed to a position between a main roller 8 and a nip roller (subroller) 9 through a film guide 10. The main roller 8 is rotated by a drive of an electric motor (not shown). The main roller 8 and the nip roller 9 grip the film 2 between themselves and feed the film 2 in the secondary scanning direction S. Under the situation, the film 2 on the main roller 8 is scanned in the main scanning direction M with a light beam from an optical system 12 on the basis of an original image data, and then, a reproduced image is obtained on the exposed film 2.
In the above exposure step, high film feeding accuracy and high positioning accuracy of the reproduced image are required. Therefore, a back tension is loaded on the film by applying load resistance to the roll film 6. And by virtue of the back tension, a contacting force between the film 2 and the main roller 8 is increased so as to keep the feeding accuracy of the film 2 and the positioning accuracy of the reproduced image.
However, oblique or diagonal running of the film 2 and weaving or meandering of the film 2 cannot be sufficiently prevented by merely applying the back tension. FIGS. 2A and 2B are views of a film 2 shifting the self position due to the oblique running and the weaving, respectively. FIG. 2A shows an oblique running of the film 2 in which the film 2 is inclined at an angle A.sub.1 of .alpha..degree. with the secondary scanning direction. Further, FIG. 2B shows the film 2 weaves where the center O' of the film 2 moves in the course of width of 2.beta. relative to the center O of the main roller 8. The line .gamma. is a locus of the center O' of the film 2.
The above mentioned oblique running and weaving of the film 2 have a great influence on the pattern of the reproduced image. For example, under the condition of the oblique running of FIG. 2A, an original image of a rectangular pattern is reproduced to a reproduced image of a rectangular pattern inclining at an angle A.sub.1 of .alpha..degree. with respect to the scanning direction as shown in FIG. 3. And if the weaving of FIG. 2B is further added to such condition, the reproduced image becomes to a warped rectangular form having curved sides 90 as shown in FIG. 4.
Under the above mentioned conditions, any normal reproduced image pattern cannot be obtained. Therefore, the film 2 is required to be returned back to the center in the main scanning direction, and should be arranged so that the film 2 is in parallel with the secondary scanning direction or has the angle A.sub.1 of zero and so that the center O' meet with the center O as shown in FIGS. 2A and 2B.
Therefore, for example, in such case that the oblique running or weaving arises, the film 2 is released at once, and is reset by feeding the forwarding portion of the film 2 toward the main roller 8 through the film guide 10. By virtue of the above-mentioned resetting, the positioning aberration or twist of the film 2 can be corrected by the self weight.
As mentioned above, in the conventional devices, aberrations due to the oblique running and weaving must be corrected by such manner during the sheet feeding.
However, such conventional sheet feeding device has problems as follows.
When the object to be fed is a short film, the influence of oblique running is not so great, even if merely initial correction by self weight is performed. For example, if an error of the initial correction causes an oblique running of 1 mm per 1 m, the aberration for feeding of a film of 1 m is in an approvable range. Therefore, such manner of initial correction does not raise any problem.
However, the oblique running provides aberration increasing in proportion to the feeding length. Therefore, for a long film, e.g. 10 m in length, the aberration due to the an oblique running becomes to an extent of 10 mm. Since the positioning aberration of 10 mm is over the approvable range, the position accuracy of the reproduced image cannot be kept well. Therefore, in a case of feeding of a long film, the initial correction should be more accurately performed.
Further, the initial correction by the self weight receives influence of the initial condition of the film 2, e.g. condition of curl of the film 2, influence of the contact with the film guide 10, and the like. Therefore, the position determined by the initial correction itself tends to disperse and lacks accuracy.
In order to eliminate the above problem, some correcting devices having more complicated mechanism might be employed. However, such correcting device rises another problem that cost and space increase.
The above mentioned problem is regarded as important in a color image setter for a color DTP(desk top publishing) device. The color image setter continuously records reproduced images for four colors, i.e. Y(yellow), M(magenta), C(cyan) and K(black) by exposure, and the size for each color image is large. Therefore, the film used for one exposing scanning is very long, and high feeding accuracy is required. Further, a small-sized color image setter like a business machine is also required.